The purpose of this research study is to evaluate the safety, efficacy, and dosing of intramuscular injections of Botox Purified Neurotoxin Complex in the treatment of arm spasticity in post-stroke subjects. Botulinum toxin has been used extensively to treat patients with muscle dystonias. This study is intended to provide more detailed information in post stroke subjects. Botox will be compared with its vehicle. There is approximately a 25% chance that a patient will be assigned to receive the vehicle. Neither the patient nor the investigator will know which medication is given, but in the event of an emergency, access to that information can be obtained. Approximately 72 subjects will participate in the study. This research study is sponsored by Allergan, Inc.